This invention relates generally to RF mixer circuits for broadband applications with improved intermodulation product suppression.
Broadband signal amplifiers are used in various wireless signal transmission applications requiring mixers for stepping up signals for transmission in assigned frequency channels and stepping down received signals to an intermediate (IF) frequency. Unfortunately, amplifiers distort signals by generating unwanted intermodulation products that can cause interference and cause poor performance.
Intermodulation distortion is defined in terms of the peak spurious level generated by two or more tones injected into a receiver. A receiver may be characterized by a third order distortion figure of merit referred to as xe2x80x9cthird order input intercept pointxe2x80x9d (IIP3), which is defined as the input power (in the form of two tones) required to create third order distortion products equal to the input two tone power. The linearity of an amplifier, and thus the immunity of the receiver to distortion and cross-talk, improves with the IIP3 of the amplifier.
Linearity and noise factor (NF) of the mixer are generally totally dominated by the transconductor stage which translates a voltage input to a current. The mixer is linearized by applying negative feedback to the transconductor, such as emitter degeneration or series feedback, which is a well-known technique. The emitter degeneration does not cancel the IM3 products, but merely surpresses them, at the expense of gain.
The present invention is directed to overcoming limitations of the prior art and providing a broadband mixer with improved intermodulation suppression.
In accordance with the invention, an RF mixer circuit having reduced intermodulation products includes a cascode transconductance amplifier including a first transistor having a control electrode connected to receive an input signal and serially connected with a second transistor having a control electrode connected to receive a bias potential. A balanced mixer is coupled to the second transistor by a first inductive element for receiving an amplified input signal, and receives a local oscillator input and produces an output signal with an intermediate frequency related to the frequencies of the amplifier input signal and the local oscillator. A second inductive element couples the second transistor to ground or a common terminal. To suppress intermodulation components, a feedback circuit is provided from the second transistor to the control electrode of the first transistor including a resistor serially connected with a capacitor, whereby the feedback circuit compensates for signal phase shift by the inductive element.
The feedback circuit components are selected as a function of input impedance of the mixer circuit, inductance of the inductive element, and the maximum frequency (fT) of the cascode amplifier. The transistors can be bipolar or field effect transistors such as MOSFETS and CMOS transistors. The balanced mixer may be a single or double balanced design with a coupled coil output.